Effect of Salt Concentration on the Corrosion Behavior of Carbon Steel in CO₂ Environment

Effect of salt (NaCl) concentration on the corrosion rate of carbon steel in CO₂-saturated solutions was explored in a wide range from 0.001 wt% to 10 wt%. Electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) were utilized to determine the instantaneous corrosion rate of C1018 carbon steel under freely corroding condition. Both EIS and LPR results indicated that corrosion rate decreases with increasing salt concentration, and the highest corrosion rate was found at the lowest salt concentration (0.001 wt%). Potentiodynamic polarization and potentiostatic hold experiments were performed to study anodic and cathodic kinetics using microelectrode technique. Cathodic polarization results showed that the cathodic reaction is under diffusion or mixed control and the limiting current increases with decreasing salt concentration. Anodic polarization results suggested that chloride adsorption prevails at high salt concentration, while hydroxyl adsorption prevails at low salt concentration. Anodic current density increases with chloride concentration and an order of 0.5 was determined. Evans diagram was proposed to explain the salt concentration effect on both anodic and cathodic kinetics. Composition and structure of the corrosion scale were examined and its formation was dissected into two stages based on corrosion rate trend and visual observation.